Enforce ClosureOutlivesSubjectTy not traversable

Previously only enforced via comment, now via negative impls.

Author: eggyal

compiler/rustc_middle/src/mir/query.rs

@@ -399,14 +399,16 @@ pub enum ClosureOutlivesSubject<'tcx> {
 /// This abstraction is necessary because the type may include `ReVar` regions,
 /// which is what we use internally within NLL code, and they can't be used in
 /// a query response.
-///
-/// DO NOT implement `TypeVisitable` or `TypeFoldable` traits, because this
-/// type is not recognized as a binder for late-bound region.
 #[derive(Copy, Clone, Debug, TyEncodable, TyDecodable, HashStable)]
 pub struct ClosureOutlivesSubjectTy<'tcx> {
     inner: Ty<'tcx>,
 }
 
+/// DO NOT implement `TypeVisitable` or `TypeFoldable` traits, because this
+/// type is not recognized as a binder for late-bound region.
+impl<'tcx, I> !ty::TypeFoldable<I> for ClosureOutlivesSubjectTy<'tcx> {}
+impl<'tcx, I> !ty::TypeVisitable<I> for ClosureOutlivesSubjectTy<'tcx> {}
+
 impl<'tcx> ClosureOutlivesSubjectTy<'tcx> {
     /// All regions of `ty` must be of kind `ReVar` and must represent
     /// universal regions *external* to the closure.